You know that movie scene in Indiana-style jungle exploration films where you stare at a rushing river until you see a primitive tribesman gangling about, sometimes on all fours, jumping from rock to rock and growling with the grace of an animal, then plunging its head into the water before pulling out with a fish in its teeth, and then it spots you and jumps across the rocks with superhuman speed before bowling over you and shrieking in your face?

These are parasites in certain rivers that turn ordinary normal humans into those guys. They were once a great race of water people- naiads- who were forced to retreat from the depths of the oceans and into the pristine streams after something something pollution(?). They fiercely guard their rivers in order to prevent their race from extinction. And this is the story of the Foundation contacting them and conducting diplomatic liaisons (and a few battles) with them.

These are parasites in certain rivers that turn ordinary normal humans into those guys

If they're parasites, why do they alter the host in such a way? Wouldn't it make more sense to go relatively undetected? Furthermore, what benefit do these parasites derive from changing their hosts so significantly?

They were once a great race of water people- naiads- who were forced to retreat from the depths of the oceans and into the pristine streams after something something pollution

Oceans are saltwater and streams tend to be freshwater and shallower and much more difficult to move around and survive in for larger species. I'm having a bit of a hard time believing this. There's a reason a lot of aquariums can't keep large sharks alive for long.

They fiercely guard their rivers in order to prevent their race from extinction.

If their race was in danger of going extinct, why did they move into an arguably more vulnerable area of water?

And this is the story of the Foundation contacting them and conducting diplomatic liaisons (and a few battles) with them.

This might be workable as a tale, but you'd definitely need to brush up on the scientific background to make sure it's believable and not cheesy for the sake of dramatics.

1. They screw with their hosts' brains because that is how they think. They ARE xenophobic tribespeople.
2. Good point. I'll move them, maybe to a previously pristine lake that got polluted?
3. When I said "certain rivers" I meant "those Man could only ever accidentally stumble upon that are otherwise hidden in the depths of the wild"
4. K

1. Because the possession bit is the final stage of their life cycle. They nest inside something or they die.
Usually this role is fulfilled by the most useful thing in the area. Apex predator fish, or large crabs. Humans have opposable thumbs, which is REALLY useful.

2. What do you mean? Rivers flow from them, so they could escape into the rivers just fine.

3. They choose the lazier ones, and anyway they usually burrow in the ground like garden eels.

1. Because the possession bit is the final stage of their life cycle. They nest inside something or they die.
Usually this role is fulfilled by the most useful thing in the area. Apex predator fish, or large crabs. Humans have opposable thumbs, which is REALLY useful.

The extreme variety between all three of those doesn't really make sense. If these things are xenophobic, why are they so willing to just mutate pretty much anything? I feel like with every answer you give, the premise just gets shakier.

2. What do you mean? Rivers flow from them, so they could escape into the rivers just fine.

Not if there's a landslide that cuts off river access, beavers build a dam, humans set up a canal, etc. That said, you seem to be getting too caught up in minutiae here. These are basically just generic merpeople, and aren't any more interesting than the ones already portrayed in popular franchise media like Harry Potter.

3. They choose the lazier ones, and anyway they usually burrow in the ground like garden eels.